doc: fix typos in README
doc: fix typos in README

 - Fixed typo: "insallation" -> "installation"
 - Added an "that" in a sentence where it was needed for clarity.

PR-URL: https://github.com/nodejs/node/pull/2852
Reviewed-By: Michaël Zasso <dc852f7f42c586356f6be32b8780273088b3bded@gmail.com>
Reviewed-By: Rich Trott <28a7b6144c43947e3f532c033ff42c8a2ae08045@gmail.com>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node